Method and apparatus for testing rock coal dust

ABSTRACT

A method and apparatus for determining the percentage incombustibles in a rock coal dust mixture by irradiating a sample of rock coal dust with gamma radiation from a collimated source spaced from the sample and measuring the backscattered radiation at an area behind the source. The sample of rock coal dust has a depth of about 2 inches to the entering gamma rays in order to minimize any changes in the count rate due to sample depth and density. Time is measured for detection of a predetermined number of radiation counts as a measurement of the percentage of incombustibles. The apparatus includes a source of radiation collimated by a lead holder to irradiate a sample of the mixture of rock coal dust. The lead holder is centered on a detector&#39;&#39;s area of sensitivity to give significant count rates with relatively small sources of radiation.

United States Patent [1 1 [111 3,735,126 Casper May 22, 1973 METHOD ANDAPPARATUS FOR TESTING ROCK COAL DUST [75] Inventor: Karl J. Casper,Cleveland Heights,

Ohio

[73] Assignee: Reuter-Stokes Electronic Components, lnc., WarrensvilleHeights, Ohio [22] Filed: Jan. 8, 1971 [21] Appl.No.: 104,937

[52] US. Cl. .....250/43.5 D, 250/83 SA, 250/83.3 D, 250/106 S [51] Int.Cl. ..G0ln 23/10 [58] Field of Search ..250/43.5 D, 83 SA, 250/106 S,83.3 D

[56] References Cited UNITED STATES PATENTS 3,448,264 6/1969 Rhodes..250/l06 S X 3,505,520 4/1970 Stewart et al... ..250/43.5 D 3,270,2048/1966 Rhodes ..250/43.5 D X 3,399,303 8/1968 Berk ..250/106 S X PrimaryExaminer-Archie R. Borchelt Attorney-Fay, Sharpe & Mulholland ABSTRACTradiation counts as a measurement of the percentage of incombustibles.

The apparatus includes a source of radiation collimated by a lead holderto irradiate a sample of the mixture of rock coal dust. The lead holderis centered on a detectors area of sensitivity to give significant countrates with relatively small sources of radiation.

16 Claims, 6 Drawing Figures PATENTEMKYZZIQYS SHEET 1 OF 2 INVENTOR.KARL J. CASPER A TTOR/VEYS BACKGROUND OF THE INVENTION This invention isan improvement of the Stewart et al device illustrated in U.S. Pat. No.3,505,520, the disclosure of which is incorporated herein by referenceas well as the references and other patents cited therein.

Coal dust fires and explosions are among the most serious hazards incoal mining. Shock waves which precede the actual combustion stir updust on the floor and walls and increase the danger. If the dust in amine has a high combustible content, i.e., coal dust, the explosion willnot be confined to a small area but will spread rapidly. General safetypractice required spreading rock dust to bring the incombustible contentof the dust on the floor and walls to the legal minimum of 65 percent.Unfortunately, rapid determination of the incombustible content had beenimpossible since chemical analysis required several days and determinedonly a condition existing in the mine at the time the sample was taken.Thus, the delay in analysis was an impediment to legal enforcement anddid not offer an adequate solution to the most important problem thesafety of the mine.

The Stewart et al patent mentioned above offered a partial solution tothe time delay problem. It found that the backscatter radiation fromgamma rays to a sample of the rock coal dust mixture indicated adetermination of the percentage of incombustibles. However, the Stewartet al method and apparatus were not entirely satisfactory. The Stewartapparatus was bulky and the relationship of backscatter radiation topercentage incombustibles was relatively complex for a simple mechanism.It is the intent of this invention to provide a portable unit operablein the mine which gives an immediate reading of the incombustiblecontent of the mine dust. In this manner it is a direct contribution toreducing the fire hazard.

The method and apparatus utilized in this invention relies upon thebasic principle established experimentally by Stewart-et al, that is, anumber of gamma rays backscattered from a sample of mine dust isproportional to the percentage of combustible content. Utilizing thisbasic concept, this invention has significantly improved the relativegeometry of the components of the equipment in order to provide improvedaccuracy and reduction in the number of components.

In particular, it was found in this invention that if the source ofradiation were centered over the detector and between the detector andthe sample, more accurate readings could be obtained because of theincrease in the solid angle of radiation impingement. This arrangementwith the use of a collimated radiation source provides a significantincrease in the amount of backscatter radiation and thus permits the useof smaller radiation sources and less insulation.

The method of determining percentage incombustibles in this inventionhas also provided a significant advantage over the prior art.Previously, the count rate of the backscatter radiation was used as adirect determination of the percentage incombustibles. This relationshiphowever was relatively complex and did not lend itself to translationwithout problems. In this invention it was discovered that thereciprocal of the count rate formed a linear relationship with thepercentage incombustibles. Thus, all computations and/or equipment werevastly simplified to translate the backscatter radiation informationinto a reading of percentage of incombustibles. It was also found thatthe reciprocal of the count rate (counts/seconds) could be measureddirectly noting the time for a given number of counts (seconds/counts).

The advantages of this method and apparatus are obvious. They provide auseful sampling procedure which gives an immediate determination of thepercentage of incombustibles of rock coal dust. The tests utilizing thismethod and apparatus can be conducted in the mine with a portableinstrument and thereby give additional mine safety. The ease and speedof sampling using this method and apparatus can vastly increase thefrequency of inspection and thereby the safety of each mine. Thestatistical error is also held fixed and can be preselected.

An additional advantage of the present method and apparatus will allowmine owners and managers who presently have no means of policing theirown mines to make their own tests. This would provide them with themeans to control the rock coal dust content within a conforming rangeand increase the safety of the mine.

SUMMARY OF THE INVENTION An apparatus for quantitatively determining thecomposition of a mixture which includes a radiation detector having anarea of sensitivity facing but spaced from a means for holding a sampleof the mixture. A source of radiation is located between the area ofsensitivity and the sample and is directed to emit radiation in thedirection of the sample so the detector senses only backscatterradiation. A means for counting the backscatter radiation is operativelyconnected to the radiation detector.

The method for determining the percentage of combustibles in rock coaldust includes radiating the sample with radiation from the source,detecting the radiation reflected by the sample and measuring thereciprocal of the count rate of the deflected radiation as adetermination of the percentage incombustibles.

PREFERRED EMBODIMENT FIG. 1 is a top plan view of the instruments ofthis invention.

FIG. 2 is a side plan view partially cut away illustrating theinstrument of this invention.

FIG. 3 is a side cross-sectional view of this invention with the sampleshown in place.

FIG. 4 is a graph of counts per minute radiation versus incombustiblesin rock coal dust.

FIG. 5 is a perspective view of the collimator for the source ofradiation.

FIG. 6 is a cross-sectional view of the collimator taken along lines 6-6of FIG. 5.

As illustrated in FIGS. 1 and 2, the apparatus of this invention is aportable instrument for use in determining the percentage ofincombustibles in rock coal dust and is illustrated generally as 10. Itis enclosed in a casing 11 having sides 12 and 13, front 14 and back 15.

A. Instrument Configuration A source of radiation 18 is held in acollimated lead block 20 which may be surrounded by an aluminum holderwith a stainless steel liner. A convenient size of the block is 0.375inches long by 0.25 inches wide by 0.25 inches high although others maybe used. The assembly of the source and holder is held by epoxy or othermeans on a brace in the middle of the counter window 22. The counterwindow is an area of sensitivity for a detector 24 which sitsimmediately below and supports the lead block 20. The lead block servesto shield the counter from the direct radiation of the source 18. Powerfor the detector 24 is supplied by a battery pack 26 mounted closelythereto.

A means 28 for measuring the count rate or reciprocal thereof includes adigital read-out 30 which may include a printer and a smallcomputer-like device 32. This means for measuring the count rate or thereciprocal thereof illustrated in general terms of this invention arenot disclosed since the details thereof do not form a part of theinvention claimed in this application.

Platform 34 forms a means for holding a sample of the mixture which isto be tested. The platform 34 generally has a lead shield 36 so that thesource of radiation 18 is not exposed until such time as desired. Thisshield 36 is naturally removed when the apparatus is in use.

An expanded view of the radiation source, detector and sampler areillustrated in FIG. 3. As noted therein and in FIG. 2, the detector 24which is known in the art and is the type sold by Reuter Stokes ofCleveland, Ohio catalogue No. SK-560, is supported by screw threadswhich are connected to the support 34 of the instrument. In this way thedistance between the detector and the sample can be adjusted.

The sample of the rock coal dust under test is held in a container 37.In order to achieve the desired accuracy of the instrument calibrated,sample cans are used. They are aluminum cans with thin bottom platesabout 0.005 inch thick which are free of dents and bubbles althoughother embodiments can be used. A marked section of the plate 34 can beused to insure correct relative positions of the components. The plate34 of the unit should be free of dust and the can must make intimatecontact with the top of the unit.

The spacing between the sample and the source which is important for theaccuracy of the instrument and must be at least initially calibrated. Inthe preferred embodiment, the spacing between the sample and the sourceshould be about 6 mm and the spacing between the sample and the detectorshould be about 14 mm.

B. Radioactive Source The radioactive source used in these measurementsis the type stated in the Stewart et a] patent noted above and inparticular AM 241 which has a half life of 458 years. Previously, asource strength of 22 millicuries was required to achieve a countingrate of 500 counts per second. While a source of this size: was not asignificant problem in a laboratory situation, it was unsuitable for aportable unit being operated in the field by personnel relativelyinexperienced in handling radioactive sources. One of the significantadvantages of this invention is that it enables a considerable reductionin the source size. This is duein part to utilizing a closer arrangementof source and detector and by placing the source in the center of thearea of sensitivity or counter face of the detector. By providing aclose arrangement of source and vdetector an increase in the countingrate was achieved because of the increase of the solid angles (shown inFIG. 3 as between the source and sample and by the increase of the solidangles between the scattering centers of the sample and the detector(shown in FIG. 3 as (b). The counting of radiation is proportional tothe product of these two solid angles.

The use of a source of radiation in the center of the area ofsensitivity of the detector as illustrated in FIG. 1, also increases thesolid angle as noted above. In addition, this configuration utilizes aspecial nature of the Compton scattering efiect. It has been found thatthe cross-section for Compton scattering is at a maximum when thescattering angle is 180, that is, when the reflected path is the same asthe incident path. At angles near 180 the effective cross-section isalso large and produces significantly increased backscattering counts.In contrast to this, previous configurations and particularly that shownin the Stewart et al patent, scattering occurred at angles approximatelywhere the crosssection of the electron is considerably smaller andpossibly a minimum. It is possible in this invention to have the sourceoff center of the detection area as long as it is between the sample andthe detector. It is important to permit about 180 deflection from thesource of the detector.

A second advantage of having a large solid angle between the source anddetector is that the energy spectrum of the scattered gamma rays have avery sharp peak in the backward direction. This spectrum is of a typewhich can be easily spanned by a discriminator window. That is smalldrifts in the discriminator setting have little effect on the overallcounting rate. If the scattered spectrum is sharply peaked, then thediscriminator window can have a relatively wide setting and small driftscan be tolerated. This advantage leads to long term stability in theinstrument which naturally is highly advantageous.

The importance of the source configuration as detailed above wasmanifested in experimental results. Counting rates of three counts persecond per microcurie of source were obscured. Thus, a counting rate of500 counts per second was obtained with a source strength of microcuriesinstead of the 22 millicuries required by the Stewart et a1 device. Thisis a factor two orders of magnitude smaller than the source used byStewart et a1.

It is probable that some of the increase in the results is from usingthe AM 241 source in conjunction with a lead holder 20 as illustrated inFIG. 5. The radioactive source AM 241 emits two gamma rays with energiesof 26.3 keV and 59.6 keV. In addition, Neptunium K x-rays are also to beobserved at 13.95 keV and 17.74 keV. In short, the close proximity ofthe lead holder to the source apparently results in a magnification ofthe source intensity through the production of lead x-rays with noincrease in the actual source strength. This is a third effectcontributing to the reduction of the source size through an increase inthe backscattered count rate.

As illustrated in FIGS. 5 and 6, the lead block holder 20 has a front40, sides 42, top 44 and a back and bottom not visible in FIG. 5. Arecess 46 in the top 44 of the lead block holder is generallyrectangular in shape and has a front 47, back 48, sides 50 and a bottom52.

The lead block 20 holding the source performs two important functions.First, it is designed to reduce the amount of direct radiation, asopposed to scattered ra diation, seen by the counter by interposing leadshielding between the source and the counter. The lead shielding must besufficiently thick that no significant error is introduced and,generally, the direct radiation is attenuated to less than 1 percent.Second, for sources commonly available and for a standard sampleconfiguration as described herein, the amount of collimation must beadjusted to yield a bulk density minimum. The source 18 is usuallyplaced in a hole about 0.100 inches below the top of the lead block. Thecross-section of the hole is determined by the size of the source.

C. Sample Another important consideration in the method and apparatus ofthis invention is the size of the sample which is irradiated. Inparticular, the thickness, that is, the depth of the sample, facing thesource of radiation has been found to be important. If a very thinsample is used, the gamma rays have a tendency to pass through thesample without significant backscattering. It was generally found thatthe intensity of the backscattered gamma rays increase with thickness upto a certain depth. Beyond this point called the infinite halfthicknessthe intensity remains nearly constant. That is, the amount ofbackscattering appears to reach a constant level.

Briefly, there are two effects which determine the thickness of thesample which is to be placed in the sample holder 36. As the thicknessof the sample increases, there is a tendency to increase the intensityof the backscattered gamma rays by an amount which varies slowly withsource sample separation distance. The Compton scattering cross-sectionfavors direct backscattering at 180 and the intensity variation dependson the reduction in solid angle at scattering angles which are lessfavored.

The second effect, however, tends to reduce the intensity of thebackscattered gamma rays as the thickness increases. The additionalmaterial is separated from the source by the other lower coal dust whichwill absorb and scatter the gamma rays. Thus, gamma rays, in order to bescattered by the additional layer, must pass through the interveningdust twice, once from the source to the upper parts of the sample andagain from the sample to the counter. At some point the attenuation ofthis intervening layer will cancel any increase in intensity that wouldhave been realized from the additional layer of material. This point ofthickness is defined as the infinite half-thickness.

It has been found that in the use of rock coal dust in this inventionthat an actual value of the infinite halfthickness is approximately 2inches. By experiments with different densities and percentage ofincombustibles, it was found that 2 inch thickness for the sample wassuccessful. Samples thicker than this infinite halfthickness produced anerror that was not significant. However, samples less than thisthickness could not produce the desired amount of backscatter necessaryto make an accurate determination of the percentage incombustibles.Slightly smaller thicknesses, however, can be utilized withoutdramatically causing error. Thicknesses less than 1 inch, however,should be avoided since they appear to significantly reduce the numberof counts. It should be understood, however, that by using differentdistances and source arrangements that different thicknesses could beused which would produce the desired results. One method of giving auniform sample is to provide a cup having a standard diameter and heightas discussed earlier.

D. Method of Determination of incombustible Content Specifically, theprocess includes irradiating a sample of the material with gamma rays.The source of the gamma rays is centered on the area of sensitivity of adetector in order to increase the amount of backscatter radiation thatcan be detected. The radiation is subsequently detected and measured. Itwas known that there was some relationship between the backscatterradiation and percentage incombustibles. The relationship, however, wasone that did not easily lend itself to a direct conversion toincombustibles.

A significant discovery of this invention was the realization that itwas not necessary to design an instrument to interpret the complexrelationship to obtain the necessary information. It was discovered thatthe reciprocal of the count rate had a linear relation with thepercentage incombustibles. This relation, shown in FIG. 4, illustratesthe linear relationship of the reciprocal of the counts per minute,i.e., minutes per count versus percentage incombustibles in rock coaldust. It should be noted that this realization made the method andapparatus for determining percentage incombustibles significantlysimpler and is an advancement in this art.

The curve of FIG. 4 shows the simplicity of the rela tionship.Mathematically this may be expressed as where P is the percentincombustibles, t is the time to reach a fixed number of counts and aand b are constants. If the extremes of percents of coal and rock dustare measured to determine the constants a and b, P (t) can be measuredfor any other sample.

The experiments have also shown that the reciprocal of the count rate islinearly related to the percentage of incombustible content and can beexpressed differently. The reciprocal of the total counting rate can beshown as a combination.

where N is the counting rate that would occur only if the coal werepresent, N is the counting rate that would occur if only rock dust werepresent, and P and P are the percentage of the coal and rock dustrespectively.

Since these quantities are linearly independent this discovery may applyto mixtures of substances other than coal and rock dust. Ordinarily, itwould be expected that the separate counting rate would be independentof each other since the intensity of the backscatter should beproportional to the cross-section and the total counting rate should beproportional to constants depending upon a relative percentage times theseparate count rate.

It should be noted that the linear independence of the components isprobably dependent upon the exclusive reaction due to Comptonscattering. If other effects take place, it would probably detract fromthe results disclosed herein. Thus, as long as there are no reactionsamong the two components used in the mixture there should be no reasonwhy the relationship discovered would not apply to other mixtures inwhich Compton scattering is used to evaluate the percentage of themixture.

It is possible to measure the count rate for a sample and then take thereciprocal of the count rate and plot the results. However, while thisis simplified from the prior art it is more complex and time consumingthan required. The reciprocal of the count rate is simply the .mannerthe time necessary for a given number of counts.

In practice, it was found that approximately 20 secends at 500 countsper second yielded a 1 percent statistical error. This is a high degreeof accuracy and is satisfactory for most uses. However, there are otheroften inaccuracies within any given instrument that would not actuallypermit a 1 percent error. Thus a longer time may be necessary in orderto collect more counts and thus further reduce the amount of error.

E. Error There are three major sources of error, they are bulk density,moisture content and variations in the bulk density minimum withincombustible content. As disclosed by the Stewart et al patent, thereis a point called the bulk density minimum at which the effect ofdensity are negligible. This point has been found to be related to theseparation distances among the source detector and sample. Variation ofthe detector sample distance in experiments showed that at separationsless than the bulk density minimum the counting rate in the low densitymaterial was higher while at distances greater than the minimum thehigher density material produced a higher counting rate. The experimentsin this invention showed that the detector-sample separation of about10-15 mm and in particular 14 mm produced a result where the density didnot create any measurable difference.

it was also found in this invention that the sourcedetector distance hadto be minimized in order to obtain a bulk density minimum. Variations insource sample distance then resulted in concurrent variations in thedetector sample separation. A successful range of distances between thesource and sample is about 2 to 7.5 mm. It was further determined that abulk density minimum could only be achieved when the source wascollimated. With a 3/16 inch lead collimator as shown in FIG. 5, theabove-noted approximate 14 mm separation produced a bulk minimumdensity.

The second error producing consideration is the moisture content of thesample. The moisture content decreases the bulk density of the materialand thus reduces the backscattering gamma ray intensity. At the sametime, the increasing hydrogen content from the water creates a higherdensity of scattering centers, increasing the backscattered gamma rayintensity. At the bulk density minimum, the counting rate shouldincrease with moisture and produce a relatively large er ror. However,the air introduced by the moisture content can be compensated over alimited range by using a source sample separation distance greater thanthe bulk density minimum.

This compromise in order to solve the problem requires a distancebetween the source and sample where the density and moisture errorsintroduce a minimum overall error. In practice, it is possible to dothis for moisture contents up to about 5 percent. Significantly, above 5percent the readings show a lower incombustible content then is actuallypresent. Thus, after compensation, the moisture content error is in sucha direction as to provide a margin of safety.

in the present invention it was found that the bulk density minimumusing a source sample separation of 14 mm actually produced a relativelysmaller error up to 5 percent moisture content. Thus, only slightcorrections were required.

The third source of error is the variation of the bulk density minimumwith different mixtures of coal and rock dust. That is the separationdistance corresponding to the correct bulk density minimum for 65percent incombustible content sample does not correspond with the bulkdensity minimum for a 50 percent or percent incombustible contentsample. The variation in the counting rate may amount to as much as 5percent over this range. It was found that the counting rate variesabout 5 percent for 50 percent incombustible content sample from that ofa 65 percent sample. Higher percentages than 65 percent incombustibleswere found to have less than 5 percent error. Thus, if mixtures havingmuch less than about 60 percent incombustibles were used the distancebetween the source and sample would have to be varied to give thecorrect bulk density minimum. However, in the field, the percentageincombustibles is normally about 60 percent and therefore does notcreate a significant problem.

It should be understood that variations in the size of the source,material used as the radiation source, and distance between componentscould be tolerated without deviating from the essence of the invention.

1 claim:

1. An apparatus for quantitatively determining the composition of asample of a mixture having more than one substance comprising:

a radiation detector means having an area of sensitivmeans forsupporting a sample of the mixture spaced from the radiation detectormeans, the area of sensitivity facing the supporting means;

means for emanating radiation from between the area of sensitivity andthe sample, the means for emanating radiation appropriately directingradiation toward the sample so that the radiation, detector means sensesprimarily only backscatter radiation from the source; and

means for counting the backscatter radiation operatively connected tothe radiation detector means as a measure of one of the substances ofthe mixture, said means for counting including means for inverting thecount rate as a measurement of the percentage of one of the substancesof the composition.

2. The apparatus of claim 1 wherein the source means is substantiallyadjacent to but spaced from the area of sensitivity of the detector.

3. The apparatus of claim 2 wherein the source means is substantiallycentered on the area of sensitivity.

4. The apparatus of claim 3 wherein the source means is collimated byand sets in a recess in a lead holder in order to yield a bulk densityminimum, said recess facing the sample which is rock coal dust.

5. The apparatus of claim 4 wherein the means for emanating radiationsits in the recess about 0.100 inches below the top of the lead holder.

6. The apparatus of claim 4 wherein the spacing of the source means,sample and detector means are arranged in order to reduce variations inthe count rate due to density and humidity.

7. The apparatus of claim 6 wherein the distance be tween the source andsample is about 2 mm to 7.5 mm and the distance between the sample andarea of sensitivity of the detector is about mm to mm.

8. A method of quantitatively determining the composition of a samplehaving more than one substance comprising:

irradiating the sample having an appropriate thickness to give accuratetests with radiation from a source spaced from the sample;

deflecting radiation with the sample;

detecting the deflected radiation in a given area with a radiationdetector at a distance from the sample, the distances between thesource, sample and detector being such that density and humidity indifferent samples do not cause a significant deviation in the amount ofdeflected radiation;

measuring the reciprocal of the count rate as a measurement of thepercentage of one of the substances of the composition.

9. The method of claim 8 wherein the sample is rock coal dust and themeasuring of the reciprocal of the count rate is done by measuring thetime necessary to reach a predetermined radiation count.

10. The method of claim 9 wherein the detecting of the radiation is donefrom a position on the opposite side of the source from the sample.

11. The method of claim 10 wherein the sample is about 2 inches thick toreduce differences in the count rate due to sample thickness.

12. The method of claim 11 wherein the radiation of the sample is donefrom the source located in the center of the detection area in orderthat the deflected radiation has a small angle of reflection.

13. The method of claim 12 wherein the distance of the source to thesample is a range of about 2 mm to 7.5 mm and the distance from thesample to the detector is about l0 mm to 15 mm.

14. The method of claim 13 wherein the radiation is directed bycollimator to the sample and not to the detector directly.

15. The process of determining the percentage of incombustibles in rockcoal dust comprising:

radiating a sample of rock coal dust with gamma radiation from aradiation source spaced from the sample, the sample presenting athickness of about 2 inches to the entering gamma rays in order tominimize any changes from thickness of the material;

deflecting the gamma radiation with the sample;

detecting the deflected gamma radiation entering the area centered aboutand behind the radiation source;

measuring the time for detecting of a predetermined number of radiationcounts as a measurement of the percentage of incombustibles.

16. The method of claim 15 wherein the process of claim 8 is a Amercium241 having a strength of about microcuries.

1. An apparatus for quantitatively determining the composition of asample of a mixture having more than one substance comprising: aradiation detector means having an area of sensitivity; means forsupporting a sample of the mixture spaced from the radiation detectormeans, the area of sensitivity facing the supporting means; means foremanating radiation from between the area of sensitivity and the sample,the means for emanating radiation appropriately directing radiationtoward the sample so that the radiation, detector means senses primarilyonly backscatter radiation from the source; and means for counting thebackscatter radiation operatively connected to the radiation detectormeans as a measure of one of the substanceS of the mixture, said meansfor counting including means for inverting the count rate as ameasurement of the percentage of one of the substances of thecomposition.
 2. The apparatus of claim 1 wherein the source means issubstantially adjacent to but spaced from the area of sensitivity of thedetector.
 3. The apparatus of claim 2 wherein the source means issubstantially centered on the area of sensitivity.
 4. The apparatus ofclaim 3 wherein the source means is collimated by and sets in a recessin a lead holder in order to yield a bulk density minimum, said recessfacing the sample which is rock coal dust.
 5. The apparatus of claim 4wherein the means for emanating radiation sits in the recess about 0.100inches below the top of the lead holder.
 6. The apparatus of claim 4wherein the spacing of the source means, sample and detector means arearranged in order to reduce variations in the count rate due to densityand humidity.
 7. The apparatus of claim 6 wherein the distance betweenthe source and sample is about 2 mm to 7.5 mm and the distance betweenthe sample and area of sensitivity of the detector is about 10 mm to 15mm.
 8. A method of quantitatively determining the composition of asample having more than one substance comprising: irradiating the samplehaving an appropriate thickness to give accurate tests with radiationfrom a source spaced from the sample; deflecting radiation with thesample; detecting the deflected radiation in a given area with aradiation detector at a distance from the sample, the distances betweenthe source, sample and detector being such that density and humidity indifferent samples do not cause a significant deviation in the amount ofdeflected radiation; measuring the reciprocal of the count rate as ameasurement of the percentage of one of the substances of thecomposition.
 9. The method of claim 8 wherein the sample is rock coaldust and the measuring of the reciprocal of the count rate is done bymeasuring the time necessary to reach a predetermined radiation count.10. The method of claim 9 wherein the detecting of the radiation is donefrom a position on the opposite side of the source from the sample. 11.The method of claim 10 wherein the sample is about 2 inches thick toreduce differences in the count rate due to sample thickness.
 12. Themethod of claim 11 wherein the radiation of the sample is done from thesource located in the center of the detection area in order that thedeflected radiation has a small angle of reflection.
 13. The method ofclaim 12 wherein the distance of the source to the sample is a range ofabout 2 mm to 7.5 mm and the distance from the sample to the detector isabout 10 mm to 15 mm.
 14. The method of claim 13 wherein the radiationis directed by collimator to the sample and not to the detectordirectly.
 15. The process of determining the percentage ofincombustibles in rock coal dust comprising: radiating a sample of rockcoal dust with gamma radiation from a radiation source spaced from thesample, the sample presenting a thickness of about 2 inches to theentering gamma rays in order to minimize any changes from thickness ofthe material; deflecting the gamma radiation with the sample; detectingthe deflected gamma radiation entering the area centered about andbehind the radiation source; measuring the time for detecting of apredetermined number of radiation counts as a measurement of thepercentage of incombustibles.
 16. The method of claim 15 wherein theprocess of claim 8 is a Amercium 241 having a strength of about 170microcuries.